NAND-type flash memory is a widely known example of a semiconductor memory device capable of storing data in a nonvolatile manner and realizing large capacity data storage. A cell array in a NAND-type flash memory is configured having NAND cell units arranged therein, each of the NAND cell units having a plurality of memory cells connected in series. The two ends of each NAND cell unit are connected via a select gate transistor to a bit line and a source line, respectively. Control gates of the memory cells in the NAND cell unit are each connected to a different word line.
A memory cell in a NAND-type flash memory includes a charge storage film (a floating gate configured from a conductive film of, for example, polysilicon or the like) formed on a gate insulating film, and a control gate formed on this charge storage film via an inter-gate insulating film. The NAND-type flash memory controls a threshold voltage of the memory cell by controlling an amount of charge stored in the charge storage film, and stores data based on a difference in this threshold voltage.
As further progress in miniaturization is made and research and development of multi-level data storage systems is advanced, there occurs a problem in the above-described kind of NAND-type flash memory, namely, spreading of a width of a threshold voltage distribution due to a write operation/erase operation being repeated on the memory cell. On the other hand, maintaining and improving data retention characteristics is also an important issue. Accordingly, it is desired to suppress spreading of the width of the threshold voltage distribution due to a write operation/erase operation being repeated on the memory cell, while at the same time maintaining good data retention characteristics.